Electrolytic recording medium



Sept. 17, 1968 R, BERMAN ETAL 3,402,109

ELECTROLYTIC RECORDING MEDIUM I Filed Dec. 12, 1966 M25 mmm M w! a WW mmM United States Patent 3,402,109 ELECTROLYTIC RECORDING MEDIUM Jay R. Berman, Rockville, Md., and Irving Lieblich, Elmhurst, N.Y., assignors to Hogan Faximile Corporation, Los Augeles, Califi, a corporation of New York Filed Dec. 12, 1966, Ser. No. 600,893 Claims. (Cl. 204-2) This invention relates to an electrolytic recording and to an improved electrolytic recording medium or paper.

Recording paper of the electrolytic type is generally marked by passing the paper between a positive eroding metal anode electrode and a negative non-eroding cathode electrode. The paper is impregnated with an electrolytically-conducting solution containing certain ingredients. When a voltage is applied between the electrodes and current is passed through the recording paper, metal ions are introduced into the paper from the anode and reacted with one or more of the ingredients of the paper impregnant to form a colored mark on the recording paper.

To be acceptable for facsimile recording purposes, the recording paper preferably should be white initially and should have the ability to produce marks having a density which is at least close in linear relationship to the current passed through the paper. The density of the marked area should be variable uniformly in a range from white for zero current flow through grays for currents of intermediate magnitude to black for currents of maximum flow. The mark preferably should be black to provide a good contrast with the white background.

There should also be a minimum of bleeding or fringing of the mark so as to provide good resolution. The unmarked paper while stored in moist condition in sealed containers for extended periods of time prior to use should remain stable without chemical decomposition of its ingredients which would cause discoloration of the paper or affect the reproducibility of results without recorder adjustment. Further, the marked recording paper should not become discolored, give off unpleasant or other odors, or be subject to the transfer of the mark to adjacent materials. The marked recording paper should be insensitive to light even after being repeatedly run through any of the conventional duplicating machines involving exposure to ultraviolet or other strong light sources.

Heretofore successful recording papers have been provided employing pyrocatechin, also called catechol, as the marking compound. While gene-rally satisfactory, some difiiculty has been had with the catechol papers because of the tendency of catechol to diffuse or transfer to and to discolor adjacent sheets of paper in contact with or in close proximity thereto. Catechol vapors also may cause discoloration of the walls and other surfaces near the recorder during the recording operation.

Prior to the development of the catetchol papers, attempts were made to provide papers utilizing a silver elect-rode to supply the metal ions which were reacted with a reducing agent to form a mark. Such papers are disclosed in Elsey Patent No. 2,063,992. Elsey teaches the use of formaldehyde in an aqueous solution of sodium nitrate and sodium hydroxide to form a mark under alkaline conditions by reduction of the silver ion to metallic silver. However, Elzey states that, although the time factor of reduction to free metal is very small with the solution described, yet during even the time that the silver ion is in solution it may diffuse away from its point of.

3,402,109 Patented Sept. 17, 1968 "ice the impregnated papers are unstable and discolor within a few days. Further, the free aldehydes of Elsey, in aqueous solution, have high vapor pressures and thus tend to produce objectional odors. Still further, the inability to maintain a fixed aldehyde concentration presents difficulty in obtaining reproducibility of results.

This invention provides an improved electrolytic recording medium which overcomes the foregoing difficulties and disadvantages. In accordance with this invention, a sheet of electrolytic recording medium is impregnated with an electr-olytically-conducting solution which contains a compound selected from the group consisting of ascorbic acid and erythorbic acid. Preferably, the pH of the solution is between about five to about nine, and in the preferred form is buffered in a pH range between about six and about 7.5.

In terms of an electrolytic recording system, the invention includes a sheet impregnated with an electrolytically-conductive solution containing a compound selected from the group consisting of ascorbic acid and erythorbic acid. A silver-containing anode is in contact with one part of the sheet, and a cathode is in contact with another part of the sheet so an electric current can be passed through the electrolyte to change its color where it is contacted by the anode.

The silver ions reelased into the paper by electrolytic erosion of the anode are reduced to metallic silver by the action of the ascorbic or erythorbic acid to leave a dense blue-black mark where the paper is contacted by the anode during the flow of electric current. A clear, unblurred image is obtained with minimum diffusion of the mark. The recording paper is stable both before and after marking, and the recorded mark does not fade, even after repeated exposure to light and heat such as occasioned by being passed through the copying machines using strong ultraviolent lamps.

A further advantage of the recording paper of this invention is that it has no perceptible odor either during or after recording. The impregnant has a low vapor pressure and thus there is good reproducibility of results. The impregnant is formulated with solid salts easy to handle and easily dissolved in water. The salts are inexpensive and readily obtainable.

These and other aspects of the invention will be more fully understood from the following detailed description and the accompanying schematic perspective drawing showing the recording system of this invention.

Referring to the drawing, a sheet of paper 10 impregnated with a conductive electrolyte containing ascorbic acid or erythorbic acid is passed between a stationary eroding anode 11 which contains silver, and a rotatable non-eroding cylindrical cathode 12 which has a prismatic platinum-iridium helical electrode wound around and projecting above its exterior surface. The electrode 13 sweeps a spot longitudinally along a knife-edge 14 of the silvercontaining electrode. Current passing between the anode and the cathode causes the silver to erode electrolytically from the anode and enter the electrolytic solution in the recording medium in the form of silver ions which are reduced to metallic silver by the ascorbic or erythorbic acid to leave a dense blue-black mark where the paper is contacted simultaneously on opposite sides by the anode and helical electrode during the flow of electrical current.

Suitable formulations for impregnating ten square feet of porous recording paper stock having relatively high wet strength and a thickness of about three mils are as follows:

Example I Ingredients Quantity Erythorbic acid g 6 Sodium nitrate g Water ml- 100 This solution had a pH of 7.3, with the sodium nitrate acting primarily as the electrolyte.

Example II Ingredients Quantity Ascorbic acid g 2 Sodium nitrate g Sodium carbonate g 2 Sodium bicarbonate g 1 Water ml 100 The sodium carbonate and bicarbonate in this solution buffered to a pH of about 7.3.

Example III Ingredients Quantity Ascorbic acid g 3 Sodium nitrate g 10 Water ml 100 The solution of Example III had a pH of six.

The pH of any of the above solutions could be adjusted and buffered to any desired value by well-known buffering agents. Formulations in the acid or base range can be made to work satisfactorily, but those with a pH in the vicinity of seven give the best over-all results with respect to clarity, density of the mark, and stability of the recorded sheet.

The pH of any of the above solutions can be buffered as high as 9.7 by including in them 5.3 grams of sodium carbonate and 4.2 grams of sodium bicarbonate. The pH can be maintained at about nine with a solution which contains 35.8 grams of sodium biphosphate and two milliliters of one N NaOH and five hundred milliliters of water.

With all of the formulations of the examples, satis factory marks were obtained. The marks were made by passing recording paper impregnated with the various formulations of the above examples through a facsimile recorder of the type shown in the accompanying drawing. The contact area of the electrodes against the recording medium was about .0001 inch.

In each case, a good black mark was obtained with spot speed in the range of twenty-three to about 135 inches per second and with current in the range of about 120 milliamperes to about 260 milliamperes at about thirtynine volts to about sixty-four volts.

The diffuse reflectance optical density, as read on a Macbeth Quantalog Densitometer, Model RD-100, for the marks ranged from about 1.02 at maximum paper speed to about 1.26 at minimum paper speed.

' The silver alloy electrode had a composition of about seventy-four percent silver, twenty-one percent copper, and four percent zinc. If the amount of silver is increased, the mark can be made darker for less current flow, but the silver electrode tends to wear faster. If the silver content is decreased much below about seventy-four percent, it wears better, but the mark is less dense.

Silver or silver alloys are the preferred anode electrodes because of the high optical density of the mark produced and because of its stable image. Silver produces univalent silver ions, which require less electrical energy than metal electrodes which produce multivalent ions. The use of silver thus generally results in the production of good density marks with a small expenditure of power. Silver alloy electrodes of the composition given above provide increased hardness, resistance to wear, and resistance to deformation without an appreciable effect on their marking qualities.

The cathode electrode may be made of any suitable conductive metal able to withstand the abrasive effects of the moving recording paper. Metals found acceptable are platinum, platinum alloys, steel, and others known in the art.

In the above examples, the reducing agent (ascorbic acid or erythoribc acid) is about two percent to about three percent by weight. However, the amount of reducing agent can be varied from a minimal amount which barely reduces enough silver to make a visible mark up to the solubility limit of the reducing agent. Moreover, various combinations of the ascorbic and erythorbic acids can be used rather than either of them alone. Typical concentrations of reducing agent can vary between about one and about ten percent by weight. The required concentration is selected to be compatible with conventional recording speeds and marking apparatus sensitivity.

As indicated previously, the sodium nitrate is used as an electrolyte. Other suitable electrolytes which can be used are the alkali metal nitrates and sulfates, and ammonium nitrate or sulfate. The concentration of the electrolyte salt can be varied as required for changing the electrical conductivity of the solution. Such variations can be from minimal amounts to the solubility limits of the salts used. Combinations of the various salts can be used, if desired.

The recording medium of this invention was prepared by impregnating porous white paper or other porous support with high wet strength with an impregnant prepared in accordance with the above disclosure. The impregnant was permitted to distribute itself even throughout the paper. Excess impregnant was removed by passing the paper between pressure rollers, so that when ready for use in a recorder, the impregnated paper had about twenty-five to fifty percent moisture by weight. To retard evaporation and keep the moisture content of the paper substantially uniform, the impregnated paper was stored in a suitable sealed container until needed.

It has also been found that various complexing agents which tie up metal ions, specifically copper and iron, are useful in the above solutions because they improve the stability of the formulations by reducing the susceptibility of the ascorbic and erythorbic acids to be oxidatively degraded by these metal ions. The addition of 0.3 gram of ethylenediaminetetraacetic acid tetra sodium salt to any of the above formulations is beneficial in this respect.

We claim:

1. An electrolytic recording medium comprising a sheet impregnated with an electrolytically-conducting solution containing a compound selected from the group consisting of ascorbic acid and erythorbic acid, the impregnated sheet including between about 25% and about 50% of moisture by weight of the sheet and a sealed container disposed around the sheet to maintain the moisture content between about 25% and about 50% by weight.

2. A recording medium according to claim 1 in which the pH of the electrolytically-conducting solution is between about five to about nine.

3. An electrolytic recording medium according to claim 1 which includes a compound for buffering the pH of the eleetrolytically-conducting solution between about six and about 7.5.

4. An electrolytic recording medium according to claim 1 which includes sufiicient sodium carbonate and sodium bicarbonate to buffer the electrolytic solution to a pH of about 7.3.

5. An electrolytic recording process for marking a sheet impregnated with an electrolytically-conductive solution containing a compound selected from the group consisting of ascorbic acid and erythorbie acid, comprising the steps of passing an electric current through said sheet while a silver containing anode is in contact with one part of the sheet, and a cathode is in contact with any other part of the sheet to produce a colored mark on the anode side of the sheet.

6. A process according to claim 5 in which the electrolytically-conductive solution has a pH between about five and about nine.

7. A process according to claim 5 in which the electrolytically-conductive solution includes a complexing agent for tying up copper and iron.

8. A process according to claim 7 in which ethylenediaminetetraacetic acid tetra sodium salt is present in the amount of between about 0.2% to 0.3%.

9. An electrolytic recording medium according to claim 1 which includes an electrolyte selected from the group consisting af alkali metal and ammonium nitrates and sulfates.

10. An electrolytic recording medium according to claim 1 which includes a complexing agent for tying up copper and iron ions.

References Cited UNITED STATES PATENTS 3,113,910 12/1963 Hepher 204 2 3,152,903 10/1964 Shepard 204-2 3,278,366 10/1966 80111616 204-2 HOWARD S. WILLIAMS, Primary Examiner.

T. TUFARIELLO, Assistant Examiner. 

1. AN ELECTROLYTIC RECORDING MEDIUM COMPRISING A SHEET IMPREGNATED WITH AN ELECTROLYTICALLY-CONTUCTING SOLUTION CONTAINING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ASCORBIC ACID AND ERYTHORBIC ACID, THE IMPREGNATED SHEET INCLUDING BETWEEN ABOUT 25% AND ABOUT 50% OF MOISTURE BY WEIGHT OF THE SHEET AND A SEALED CONTAINER DISPOSED AROUND THE SHEET TO MAINTAIN THE MOISTURE CONTENT BETWEEN ABOUT 25% AND ABOUT 50% BY WEIGHT. 